1. Field of the Invention
This invention relates to an ophthalmic diagnostic method and apparatus, and more particularly to a method and apparatus for measuring the length of the optical axis of an eye, i.e., the distance between the corneal surface and the eye fundus of an eye undergoing ophthalmic examination.
2. Description of the Prior Art
The use of ultrasound in instruments for measuring the length of the optical axis of the eye is known in the prior art, and such instruments have come into general use as commercial products. Another measurement method that can be employed for such an application is one based on optical interference.
The ultrasonic method consists of transmitting a beam of ultrasonic waves into the eye and receiving the reflections of the ultrasonic waves from the eye, and obtaining the distance of boundary surfaces in the eye on the basis of delays in the reflected waves. There are a number of problems with this method, which are described below.
1. The precision of the measurement of the length of the optical axis of the eye is low, being in the order of 0.1 mm.
2. To carry out the measurement, a probe that includes the ultrasonic oscillator has to be brought into contact with the eye. Although either of two variations may be used, the contact method or the immersion method, both impose a fairly considerable strain on the person being examined.
3. There is a difference between the length of the optical axis of the eye as measured by ultrasonic waves, and the length of the optical path of the eye axis.
The use of the optical interference method to measure the length of the optical axis of the eye is one way of making up for the drawbacks of the ultrasonic method.
One such method is that disclosed in German Patent Publication No. 3,201,801. It comprises directing a beam of partially coherent light into the eye and extracting two beams of light from the light reflecting from the various boundaries in the eye, the two beams of reflected light used usually being light reflected by the corneal surface and light reflected by the retina. The two beams are guided into a Michelson interferometer. On one arm of the interferometer is a fixed mirror that only reflects light reflected by the corneal surface, and on the other arm is a movable mirror that reflects both of the beams of reflected light. When the beams of reflected light from the two arms are combined for observation while the movable mirror is moved, interference fringes will appear twice. By reading off the positions of the movable mirror at the points at which the fringes appear, the length of the optical axis of the eye can be determined from the difference between the readings.
Thus, in accordance with this method the length of the optical axis of the eye is measured on the basis of interference between beams of partially coherent light, and as such the length of the optical path of the eye axis can be determined, while another merit of the method is that as the measurement is accomplished without physical contact with the eye, it does not impose any burden on the patient. However, there are the following problems.
1. The measurement procedure requires that the movable mirror be moved by mechanical means, which increases the structural complexity of the apparatus and adversely affects the stability, rendering it unsuitable for clinical applications. In addition, it is difficult to maintain a satisfactory level of precision in the movement of the movable mirror that determines the measurement precision. Also, during the measurement procedure the examiner has to confirm the interference fringes visually, which introduces a further element of imprecision into the measurement results.
2. A visible light has to be used because an observer has to view the interference fringes directly by eye, which dazzles the patient.
3. The patient has virtually no sensation of light if a semiconductor laser is used that operates in the near-infrared region, but interference fringes have to be viewed via an infrared scope or the like, which makes the apparatus complex and costly.
4. It takes at least two or three seconds to carry out the measurement, and any movement of the patient's eye during that time results in measurement errors.